1. Field of the Invention
This invention relates to a lens for a beam of charged particles that enables aperture (spherical) aberration control.
2. Prior Art Statement
Most charged particle beam lenses for use in electron and ion beam apparatuses have been of the type constituted as magnetic or electrostatic lenses whose configuration is rotationally (axially) symmetrical with the optical axis (Z axis), where the optical axis is defined as the center of the charged particle beam trajectory. Such axially symmetrical lenses do not allow correction for aperture aberration (spherical aberration).
On the other hand, in the quadrupole lens, which has four electrostatic or magnetic poles placed in parallel with the optical axis, a strong lens effect can be obtained owing to the fact that the main component of the electrostatic or magnetic field acts perpendicularly to the beam axis of charged particles. In its single stage configuration, however, the quadrupole lens is extremely asymmetrical, because it acts as a convergent (convex) lens in the XZ-plane and as a divergent (concave) lens in the YZ-plane.
Therefore, when the quadrupole lens is used for beam convergence in an ion microprobe mass analyzer, electron and ion beam lithography system, Auger electron spectroscopy system, x-ray microanalysis system or the like, it has to be combined with like lenses in two, three or more stages.
The characteristics and throughput of the aforementioned charged particle beam apparatuses improve in proportion as the beam current value rises when the beam is converged to a minute diameter. Insofar as there are no problems regarding the source size and radiation current, the beam current can be increased solely by enlarging the effective beam converging angle.
The size of the beam converging angle is, however, limited by the aperture (spherical) aberration of the lens. The blur .DELTA.r caused by aperture aberration at the image position of a magnetic lens, cylinder lens, three-electrode lens or the like which is rotationally symmetrical to the beam axis can be expressed as EQU .DELTA.r=C.sub.S .beta..sup.3 ( 1)
where .beta. is the beam convergence angle and C.sub.S is the spherical aberration coefficient.
The aperture aberration of a quadrupole lens with non-rotationally symmetrical lens action is more complex than that of the aforesaid rotationally symmetrical lens. If the beam convergence angles in the XY-plane and the YZ-plane of the quadrupole lens system are defined as .alpha. and .beta., respectively, the aperture aberration coefficient, which is proportional to the third power of the beam convergence angles, .alpha..sup.p .beta..sup.q, can be expressed as C.sub.Apq and involves four terms. Specifically, in the XZ-plane the aperture aberration coefficient becomes C.sub.A30 for p=3, q=0 and C.sub.A12 for p=1, q=2, and in the YZ-plane becomes C.sub.A21 for p=2, q=1 and C.sub.A03 for p=0, q=3. The blurs .DELTA.X, .DELTA.Y caused by the aperture aberrations can be expressed as EQU .DELTA.X=(C.sub.A30 .multidot..alpha..sup.2 +C.sub.A12 .multidot..beta..sup.2).alpha. (2) EQU .DELTA.Y=(C.sub.A21 .multidot..alpha..sup.2 +C.sub.A03 .multidot..beta..sup.2).beta. (3)
In a lens system obtained by combining three or more quadrupole lens stages and used in the same manner as a rotationally symmetrical lens, if the XZ-plane lens magnification .vertline.Mx.vertline. and the YZ-plane magnification .vertline.My .vertline. are both equal to unity, then since it follows that .alpha.=.beta., it also follows that the coefficients C.sub.A21 and C.sub.A12 are equal. This is called the stigmatic condition.
It is a principle of the lens design that the lens characteristics improve as the aperture aberration becomes smaller. It is therefore important to reduce the aperture aberration coefficient as far as possible. The reduction in the aperture aberration coefficient can be attained by the excitation control which is facilitated under the aforementioned stigmatic condition in the lens system.
It is generally possible to correct for aperture aberration in a quadrupole lens system by introducing an octupole lens into the system. Moreover, Japanese Patent Application Publications Sho 53-30628 and Sho 55-28179 teach that correction of aperture aberration can be achieved in a quadrupole lens system having no round lens effect, even without introducing an octupole lens. Specifically, these publications teach that the correction can be realized by introducing aperture electrodes to have the same optical axis as the quadrupole lens system and creating an octupole lens effect by exciting the aperture electrodes.
In the three-stage quadrupole lens systems disclosed in these prior art references, however, the coefficients C.sub.A12 and C.sub.A21 are not equal if the quadrupole lens excitation condition is not laterally symmetrical at the second quadrupole lens. Therefore, it becomes impossible to correct the four aperture aberration coefficients exactly the same.
In other words, since, for example, correcting coefficient C.sub.A30 increases the coefficients C.sub.A12 and C.sub.A21 while correcting the coefficients C.sub.A12 and C.sub.A21 increases the coefficients C.sub.A30 and C.sub.A03, it becomes necessary to cancel out very large coefficients against each other. It is therefore necessary to induce a strong octupole lens effect for this purpose. Since a small amount of remaining misalignment therefore produces a large aberration and also owing to mechanical aberration caused by misalignment, the correction characteristics are markedly degraded.
By way of example, consider the correction system taught by Japanese Patent Application Publications Sho 63-9340 and illustrated FIGS. 6(a) and 6(b). The system consists of a combination of three quadrupole lens stages 1, 2 and 3 and two aperture electrodes 4 and 5. The potential distribution of the system is shown in FIG. 7, in which curve X is the beam trajectory in the XZ-plane, curve Y is the beam trajectory in the YZ-plane, and reference numbers 31, 32 and 33 designate the potential distributions of quadrupole, octupole and round lens components, respectively. In this correction system, since the aberration correction is performed under excitation conditions wherein the quadrupole lens system comprises three stages (for a detailed explanation of the potential distribution of these lenses, see U.S. Pat. No. 4,075,488) and C.sub.A12 .noteq.C.sub.A21, there remains the problem that there cannot be realized a perfect correction system enabling free correction control of the aperture aberration coefficients.
In the same correction system, as is clear from the excitation strength (expressed as the ratio of applied voltage to accelerating voltage and indicated above the quadrupole lens system and the aperture electrodes in FIG. 7), the charged particle beam can be converged to a line near the beam entrance end of the second quadrupole only if the first-stage quadrupole is strongly excited. However, strong excitation of the first-stage quadrupole gives rise to problems such as that at the second quadrupole the separation of the beam from the axis in the XZ-plane (divergence-convergence-divergence) becomes large.
The present invention was accomplished in the light of the foregoing circumstances and has as its object to provide an axially symmetrical lens for a charged particle beam that enables control of correction and control of spherical aberration.